1. Field of the Invention
The invention relates to an optical fiber as an optical waveguide for single-mode operation.
2. Description of the Related Art
Optical fibers are used in fiber-based laser systems, for example as optically pumped amplifier fibers. Fiber lasers fulfill varied tasks, for example in high-precision material processing or also in measurement technology. Their advantages are great power density as well as high beam quality, both in pulsed operation and cw operation. Refraction limitation in fiber lasers also has great significance. This is advantageous in the case of material processing, for example, where precision is important.
Limiting factors in the development of fiber lasers are non-linear effects such as Raman scattering and self-phase modulation, for example. These are proportional to the power density in the fiber and the fiber distance traveled. A reduction in non-linear effects can be achieved by means of increasing the size of the light-guiding fiber region, i.e. the diameter of the fiber core. The design of corresponding large-core fibers is therefore aimed at preferentially propagating the fundamental mode in the fiber (so-called single-mode operation).
Various variants of large-core fibers that work in single-mode operation are known from the state of the art.
For example, DE 198 28 154 A1 describes the use of a conventional large-mode-area (LMA) fiber, in which only a small number of modes propagates at an increased diameter. To suppress higher transversal modes, the fiber is bent in targeted manner, and, at the same time or alternatively, a mode selection is carried out by means of optimized excitation.
US 2006/0176911 A1 describes a fiber in the so-called “rod-type” design. Here, the fiber consists of a central core and a mantle that surrounds it, which guides the pumped light. The mantle has a photonic structure composed of capillaries spaced apart from one another and disposed in regular manner, which run parallel to the core. Another sheath having a lower index of refraction is disposed outside of the mantle. In total, the diameter of the fiber amounts to between 1 mm and 10 mm. The fiber length amounts to less than 1 m.
Furthermore, solutions are known in which a central fiber core has at least one helical core wound around it (for example U.S. Pat. No. 7,424,193 B2). Here, the fundamental mode propagates in the central core, while the modes of a higher order couple into the helical core and propagate there, or are attenuated by means of losses, respectively.
U.S. Pat. No. 7,280,730 B2 describes hollow fibers having a hexagonal hole structure in the cladding. The structure is designed in such a manner that the ratio of hole diameter to hole spacing is greater than 0.35. A core that remains free is formed in the center of the hexagonal structure; this core is large enough to reduce non-linear effects to an acceptable dimension.
In DE 10 2006 023 976 A1 and US 2006/0263024 A1, respectively, the goal of obtaining fibers having large cores, which guarantee single-mode propagation, is pursued. The fibers have hole structures in the mantle, whereby the principle is to dispose the largest possible holes as close to one another as possible in the mantle.